Solid oxide cells (SOCs) hold great promise for clean energy conversion, yet conventional cathodes such as La_0.6Sr_0.4Co_0.2Fe_0.8O_3–δ (LSCF) suffer from insufficient electrocatalytic activity and poor CO₂ tolerance. This study designed a high-entropy perovskite, La_0.2Sr_0.2Pr_0.2Nd_0.2Ba_0.2Co_0.2Fe_0.8O_3–δ (HELSCF), via A-site high-entropy modification of LSCF. By regulating the synthesis temperature, two distinct crystal structures were achieved: an asymmetric tetragonal phase (HELSCF-Pbnm) with enhanced lattice distortion obtained at 1000 °C and a symmetric cubic phase (HELSCF-Pm3m) obtained at 1100 °C. Comprehensive characterizations confirmed that HELSCF-Pbnm exhibits superior properties, including a higher specific surface area, increased oxygen vacancy concentration, and optimized electronic structure. At 750 °C, the HELSCF-Pbnm-based symmetric cell delivers the lowest area-specific resistance of 0.040 Ω·cm², along with excellent bifunctional activity toward both the oxygen reduction reaction and oxygen evolution reaction, as well as outstanding tolerance under CO₂-containing atmospheres. When employed as the cathode in a single cell, it achieves a maximum power density of up to 1.38 W·cm⁻², approximately 1.7 times that of LSCF. Furthermore, it demonstrates exceptional operational stability for over 260 h at 600 °C. Density functional theory calculations further reveal that the orthorhombic structure enhances O₂ adsorption and d–p orbital hybridization, synergistically boosting catalytic performance. A temperature-modulated high-entropy strategy offers a facile and effective route for developing high-performance, CO₂-tolerant cathodes for reversible SOCs.